This invention relates to opto-electronic components and in particular to processes for use in manufacturing high performance components in opto-electronic hybrid module form, especially silicon opto-hybrid form.
The advantages of optical interconnects in telecommunications systems have been thoroughly demonstrated. However, as the interconnect length reduces to tens of centimeters or less, such as in computer systems and referred to hereinafter, these advantages are harder to realise. One reason for this is that optical components have traditionally been packaged separately from electronic devices, which results in a relatively low density of integration. In order to increase interconnection capabilities, new ways of packaging optical, opto-electronic and electronic components will be required.
One approach is the monolithic integration of opto-electronic components, however the monolithic integration of arrays of laser with electronic multiplexer and logic circuits presents extreme technological difficulties which are unlikely to be overcome for some years.
Another possibility is to make use of hybrid multi-chip modules. Hybrid mounting techniques allow the independent optimisation of the individual optical and electronic components. For example, InP or GaAs lasers can be mixed with silicon integrated circuits and InGaAs or Si photodetectors. In this way complex and densely packed integrated opto-electronic subsystems can be manufactured which make the best use of each technology.
A particular application of opto-hybrid technology in multiprocessor mainframe computers is that of a wideband optical bus. This is an optical implementation of a time division multiplexed bus and is illustrated in FIG. 1. Each of a number of nodes (electronic circuit boards) 1, in this case eight nodes, are interconnected by central star coupler 2 and multi-element optical ribbon fibre cable 3. At each node there are array transmitter (4) and receiver (5) modules. The whole acts as a broadcast network; all the data transmitted at one node is received by all the others. The basic network shown could operate at up to 32 Gbit/s and multiple instances would be combined in parallel to achieve the overall throughput required. It will be appreciated that this is only an example. There are many other applications in computers, telecommunications systems and elsewhere.
A number of components are required to build such a network; array transmitter and receiver modules, the passive optical star coupler and optical ribbon fibre cable. Attention is directed to our co-pending GB Application No 8925539.2 (Serial No 2237949) (J. W. Parker 5) which relates to various aspects of such a network, in particular the coupler and the transmitter and receiver modules and discusses realisations of the latter in silicon opto-hybrid form, which form can achieve the necessary compactness and reliability at a reasonable cost.
The layout of a transmitter module 4, is shown schematically in FIG. 2. Each module comprises a twelve element laser array 6, a twelve element laser driver array 7 and a twelve element MUX array 8 whereby several lower speed inputs can be combined into one data stream. Fifteen parallel low bit rate data inputs are indicated at 9 but in practice the number may be very much higher. Ten of the twelve outputs A (ten of fibres) are data outputs at say 3.2 Gbit/s, whereas one of the other outputs is used to transmit a common clock and the other is for control purposes, such as for transmitting parity information. Electrical interconnects are not shown in FIG. 2, but will for example be at B. The light from the rear facet of the laser array is monitored by an array of backface detectors (not shown in FIG. 2) whose output signals provide a bias to the respective laser driver to ensure that the lasers are kept at threshold. The output from the lasers is coupled into the ribbon fibre 3 either directly or by the use of ball lenses. The receiver module has a similar form of layout to the transmitter and can be fabricated by identical technology.
A basic transmitter module in silicon opto-hybrid form is described in GB Application No 8925539.2 (Serial No 2237949) and is shown in FIG. 3. For simplicity and clarity only one laser, one fibre and the associated driver and multiplex chips are shown. The electrical connections are omitted. The module involves a silicon substrate 20 in which a V-groove is provided. In the case of a (100) silicon substrate anisotropic etching using etching techniques mentioned hereinafter will produce V-shaped wells with the plane side walls formed by the (111) planes. With appropriate masking the groove can be made open at one end for reception of the fibre 19 and closed at the other end as indicated. Also etched in the substrate and aligned with groove 21 is a well 21' with an inclined end wall which provides a reflector, that can be metallised to improve reflectivity as can the well walls adjoining it, whose purpose will be apparent from the following. Mounted in alignment with the groove 21 and well 21' is a laser chip 23. The depth of the groove is such that the core of the fibre 19 is aligned with the output of the laser chip 23. The laser chip 23 has its electrical contacts on its face adjacent the substrate, as have the driver/multiplexer chips 24 and 25, which may be of silicon or gallium arsenide. These chips are electrically and thermally contacted to the substrate 20 using bumps of solder on photolithographically defined pads. This is by the so-called self-aligned solder bump technology in which surface tension pulls the chips into alignment to an accuracy of the order of 0.5 .mu.m. The photolithographically defined pads form part of the electrical connections referred to above which may involve one of the so-called HDI (high density electrical interconnect) technologies, for example interconnects using multiple level of polyimide and a metal. A monitor photodiode chip 26 is mounted to monitor the output from the back face of the laser chip 23. This chip 26 too has its electrical contacts on its face adjacent the substrate and also its active area which performs detection. The side walls and end inclined wall of the well 21' serve to reflect light output from the back face of the laser chip to the active area. The receiver module is similar to the transmitter module. In that case the laser chip is omitted and the V-groove extends part way under the photodetector chip, corresponding to the monitor photodiode. The chips 24 and 25 would in this case comprise demultiplexer and other functions required at the receiver module.
One of the aims of the present invention is to provide a practical process for producing the modules referred to above.